I admit my lack of knowledge on the topic, but I can quote this report: "High dependence on availability of sheep wool as the only source for commercial production act as another restraining factor which is likely to affect lanolin market price trends."
(Lanolin is the sebaceous secretion from wool from which the vitamin is obtained)
As of 2010, The vitamin D3 average price was 50$/MIU (million international units) and the market volume was 97.3 ton (Approx. 20% human consumption and 80% for animal farming). At 500,000 IU per gram, the production of vitamin for human consumption was enough to cover the daily value (800 IU/day) for 0.44% of the entire human population (7.5 billion). If you wanted to give a higher dose of around 10,000 IU/day, you could only do it for 0.04% of the population. This means the current production levels would need to be increased between 225-2800 times the current capacity to fulfill everyone’s requirements, or 45-560 if you reroute the production for animals to humans.
In my opinion scaling up the current industrial production of vitamin D3 supplements to fulfill the global needs seems infeasible, especially given the current production process based on sheep wool (though industrial extraction from lichen is a promising alternative for scale up). Another option would be promoting vitamin D rich foods like nori algae, but according to my own research nori algae production is hardly going to go up. In Japan (the top producer) one person told me that the seaweed farmers are the only ones allowed to produce it and are limiting the supply to keep the price up. Another one told me most good production sites are in operation already. We would have to start producing it somewhere else.
It may be easier to try convincing people to get more sun. You can get 10,000-20,000 IU of vitamin D from 30-180 minutes of sun exposure in a sunny day depending on your level of skin pigmentation. This would suffice for most people when feasible.